EUV light beams with fractional orbital angular momentum driven by high-order harmonic generation and conical refraction

Abstract

The study of light beams carrying integer orbital angular momentum (OAM) has become ubiquitous during the last two decades because of their unique capability to harness light matter-interaction with applications in different fields such as optical communications, super-resolution imaging, and quantum optics [1]. Alternatively, light beams carrying non-integer OAM have also recently been demonstrated. In particular, it has been demonstrated that half-integer OAM beams can be easily generated by means of conical refraction (CR) produced with optically biaxial crystals, with applications in optical trapping, free-space optical communications, material processing, and super-resolution imaging [2]. However, these applications are limited to the VIS-NIR domain due to the spectral bandwidth at which biaxial crystals are transparent (typically 400 nm–3000 nm). Therefore, new generation mechanisms of fractional-OAM beams at shorter wavelengths — such as the EUV and soft x-rays-are needed to extend the applicability of these techniques to the nanometer scale. High-order harmonic generation (HHG) stands as a unique frequency up-conversion process for the generation of coherent EUV and soft x-ray radiation, emitted in the form of attosecond bursts. A remarkable aspect of HHG is its natural ability to map the characteristics of the driving field to the high-frequency harmonics, this being fundamental in the generation of highly charged OAM beams in the EUV/x-ray domain [3-5].